This invention relates generally to the manufacture of fiber-reinforced, plastic articles such as tanks or the like and, more particularly, relates to a novel method and apparatus for manufacturing fiber preforms for such articles.
In the manufacture of fiber-reinforced articles such as tanks and the like, one commonly used method includes the steps of forming two end preforms from fibers, such as glass fibers, and positioning them in a cylindrical mold with a rolled mat used for forming the tank sidewall. The fibers forming the preform and mat are then compressed by inflating a bag within the mold and then the fiber form is partially permeated with a thermosetting resin or the like. The bag is subsequently expanded by further inflation to compress the fiber form in such a manner as to distribute the resin through the fibrous form and achieve the results of pressure molding, while at the same time avoiding migration of the fibers and destruction of the laid-up form.
The above-described procedure is set forth in U.S. Pat. No. Re. 25,241 to Randolph. In the Randolph patent, the fiber form comprises a flat sheet of fiber matting which is wrapped into a cylindrical form to define the sidewall of a cylindrical tank. The top and bottom portions of the fiber form comprise preformed fiber caps. According to the teachings of Randolph, therefore, a longitudinal overlap or seam is formed by the sidewall matting and such a seam results in an unevenness in the amount of fiber-reinforcing material along the longitudinal, overlapping portion of the matting. Further, since the end preforms are separately formed, an overlappingjoint is required between the sidwall mat and the preforms. Here again, an irregularity in the amount of reinforcing material is present at the overlapping joints. Still further, the lapping of the joints does not provide a uniform interlocking of the fibers in the area of the lap.
In order to overcome the above problems, a technique was disclosed for forming a fiber preform having a tubular section and an integral end thereon wherein the fibers forming the preform are uniformly distributed over the entire preform so that the final product provides a uniform high strength without excessive amounts of fibers in any location, and so that a finished article has not more than one seam. This technique is set forth in U.S. Pat. Nos. 3,654,002 and 3,736,086.
According to the procedure of those patents, an elongated screen form is rotated about its longitudinal axis. A vacuum is supplied to the interior of the screen form to draw air into the form while a large number of randomly oriented chopped pieces of fiber are directed against the rotating form by a roving cutter which traverses the length of the form. The cutter is followed by a thermosetting resin binder spray to lock the fibers in place. The binder-impregnated fibers are then subjected to heat to cure the binder and to produce a preform having a cylindrical sidewall and an integral end cap. Although this technique overcomes the problems associated with a plurality of lap seams, and reduces the expense of the form by eliminating the need for sidewall matting, it has been found that a considerable portion of the fibers is lost, since the screen form does not capture all of the fibers on its sidewall.
Probably as important is the fact that it is impossible to maintain a uniform wall thickness, on the preform. The chopped fiber is initially deposited by the roving cutter on the portion of the screen nearest the applied vacuum, and then the cutter traverses the longitudinal extent of the preform screen until it deosits fibers on the closed end of the screen. Monometer readings along the extent of the screen indicate a drop in vacuum as the cutter approaches the closed end of the screen, and while the deposited fibers tend to promote suction through the perforations which are not yet deposited with fibers, the fibers tend to be compacted more at the open end of the preform. Thus, while the amount of fibers is uniform at any given location along the sidewall of the preform, the thickness of the preform varies due to differences in compacting pressures. When such a preform is laid up within a mold and impregnated with resin, the fiber-to-resin ratio varies from a relatively high value at the open end of the tank to a relatively low value at the closed end of the tank. The desired objective in producing a tank or pressure vessel is to have a uniform glass-to-resin ratio throughout the tank or vessel, such as for example 40% fibers and 60% resin.